Expandable forming drum, and process for building tyres

ABSTRACT

A toroidal forming drum and a process for budding tyres. The toroidal forming drum is expanded within a shaped carcass sleeve for supporting the carcass sleeve against an abutment surface externally provided by the forming drum. An elementary semi--finished product is applied around the shaped carcass sleeve by pressing the elementary semi-finished product toward the abutment surface. The abutment surface has circumferential rows of solid portions alternated with hollow portions. The solid portions, arranged along axially opposite circumferential edges of the abutment surface, have a transverse size between 10% and 60% of a transverse size presented by the solid portions arranged in proximity to an axial centreline plane of the abutment surface.

The present invention relates to a forming drum, and a process forbuilding tyres for vehicle wheels.

More particularly, the invention is directed for the building of greentyres, to be subsequently subjected to a vulcanisation cycle forobtaining the final product.

For the purpose of the present description, with the term “elastomericmaterial” it is intended to indicate a composition comprising at leastone elastomeric polymer and at least one reinforcement filler.Preferably, such composition further comprises additives such as, forexample, a cross-linking agent and/or a plasticiser. Due to the presenceof the cross-linking agent, via heating, such material can becross-linked, so as to form the final manufactured product.

By “tyre for two-wheel vehicles”, in particular motorcycles, it isintended a tyre whose curvature ratio is as an example comprised betweenabout 0.15 and about 0.45.

By “curvature ratio” relative to a tyre (or to a portion thereof) it isintended the ratio between the distance of the radially external pointof the tread band (or of the external surface) from the line passingthrough the laterally opposite ends of the tread itself (or of theexternal surface itself), measured on a radial plane of the tyre (or ofsaid portion thereof), and the distance measured along the chord of thetyre (or a portion thereof) between said ends.

By “curvature ratio” relative to a forming drum it is intended the ratiobetween the distance of the radially external point of the externalsurface of the drum from the line passing through the laterally oppositeends of the drum itself, measured on a radial plane of the drum, and thedistance measured along the chord of the drum between said ends.

The terms “radial” and “axially” and the expressions “radiallyinternal/external” and “axially internal/external” are used withreference to the radial direction of the forming drum used/of the tyre(i.e. to a direction perpendicular to the rotation axis of the aforesaidforming drum/tyre) and to the axial direction of the forming supportused/of the tyre (i.e. to a direction parallel to the rotation axis ofthe aforesaid forming drum/tyre). The terms “circumferential” and“circumferentially” are instead used with reference to the annularextension of the aforesaid forming support/tyre. A plane with respect toa forming drum or to a tyre is defined “radial” when it contains therotation axis of the forming drum or of the tyre, respectively. By“elementary semi-finished product” it is intended a continuous elongatedelement made of elastomeric material. Preferably such continuouselongated element can comprise one or more textile and/or metalliccords. Preferably such continuous elongated element can be cut to size.

By “component” or “structural component” of a tyre it is intended anyone portion thereof capable of carrying out its own function or a partthereof. For example, components of the tyre include the liner, theunder-liner, the sidewall inserts, the bead cores, the filler inserts,the anti-abrasive, the sidewalls, the carcass ply/plies, the beltlayer(s), the tread band, the underlayer of the tread band, theunder-belt inserts etc.

A tyre for vehicle wheels generally comprises a carcass structurecomprising at least one carcass ply having respectively opposite endsengaged with respective annular anchoring structures, integrated in thezones normally identified with the name of “beads”, having an internaldiameter substantially corresponding with a so-called “fitting diameter”of the tyre on a respective mounting rim.

The carcass structure is associated with a belt structure which cancomprise one or more belt layers situated in radial superimposition withrespect to each other and with respect to the carcass ply, havingtextile or metallic reinforcement cords with crossed orientation and/orsubstantially parallel to the circumferential extension direction of thetyre (at 0 degrees). In radially external position with respect to thebelt structure, a tread band is applied, it too made of elastomericmaterial like other semi-finished products constituting the tyre.

Respective sidewalls made of elastomeric material are also applied inaxially external position on the lateral surfaces of the carcassstructure, each extended from one of the lateral edges of the tread bandup to the respective annular anchoring structure to the beads. In thetyres of “tubeless” type, an air-impermeable covering layer, normallytermed “liner”, covers the internal surfaces of the tyre.

Following the building of the green tyre, actuated by means of theassembly of respective components, a moulding and vulcanisationtreatment is generally executed, aimed to determine the structuralstabilisation of the tyre by means of cross-linking of the elastomericcompositions as well as to impart on the same, if requested, a desiredtread pattern and possible distinctive graphic marks at the sidewalls ofthe tyre.

The carcass structure, generally in sleeve form, and the belt structureare generally made separately from each other in respective workstations, in order to be mutually assembled at a later time.

WO 2004/041520, on behalf of the same Applicant, describes a shapingdrum carried by a robotic arm which interacts with a transfer membercarrying the belt structure picked up by an auxiliary drum, in order todetermine the coupling between the carcass structure and the beltstructure. The robotic arm then carries the shaping drum in proximity todevices for applying the tread band and/or the sidewalls comprisingdispensing members arranged to deposit a continuous elongated elementmade of elastomeric material on the mutually coupled carcass structureand belt structure.

The document WO 2004/041522 illustrates a further embodiment in which ashaping drum carried by a robotic arm is moved in order to interact withdevices that complete the manufacturing of the green tyre after havingdetermined the application of a belt structure previously formed on anauxiliary drum.

US 2009/0020200 describes the manufacturing of a tyre for two-wheelvehicles, in which a tread band is obtained by continuously winding as aspiral a continuous elongated element made of elastomeric material inthe circumferential direction of the tyre being processed, supported bya rigid drum whose external surface profile mirrors the internal surfaceprofile of the tyre being processed.

US 2013/0075041 proposes applying the crown structure of a tyre on acarcass structure shaped and supported by a forming drum arranged at itsinterior. The forming drum has a pair of bead grip half-parts, radiallyexpandable for determining the engagement of the carcass structure,which is shaped according to a shape very close to the structure of afinished tyre, with a mutual approaching of the half-parts themselves.Also present in the drum is a central forming unit having two series offorming plates which in an expanded condition mutually mate in order todefine a continuous 360° surface that supports the carcass structure.The plates belonging to one series and the other are mutually alternatedand in a contracted condition the plates of one series radiallytranslate inside the plates of the second series.

The Applicant proposes significantly improving the quality of theproduct by optimizing the deposition of single components of the tyrefor the purpose of building the same.

For such purpose, the Applicant has observed that by depositing anelementary semi-finished product fed by a dispenser directly onto thetoroidal forming drum, while the latter is suitably moved at thedispenser itself, it is possible to very precisely distribute theelementary semi-finished product so as to form a desired structuralcomponent of the tyre (e.g. a belt layer, a tread band or a sidewall)with greater precision than that normally attainable when conventionalsemi-finished products are used with pieces cut to size.

Nevertheless, the Applicant has perceived that the actuation of theseprinciples on an expandable drum, as for example described in US2013/075041, would be precluded, or at least obstructed, by the currentimpossibility of arranging a drum sufficiently light and manageable forthe purpose of its transfer and movement in one or more work stationsset to form the single components.

For such purpose, the Applicant has observed that the use of plates withcontinuous abutment surface obliges the subdivision of the plates intotwo separate series, separately movable at respective subsequentmoments, in order to prevent mutual interference and jamming during thecontraction and expansion of the drum. Consequently, complex drivingmechanisms are required, which considerably increase the overall weightof the drum. The need to separately move the plates, carrying a seriesof plates inside the other in the contracted condition, also makes itdifficult to confer a diameter size to the drum that is sufficientlylimited in the contracted condition.

The Applicant has perceived the possibility to attain a significantlightening of the drum if in place of a continuous abutment surface,even if apparently optimal for the purpose of manufacturing the tyrecomponents, sectors are used that are mutually interconnected atrespectively complementary cavities. According to the Applicant, thelightening will not be due exclusively to the lower weight determined bythe lack of material at the cavities, but also and above all due to asimplification of the driving mechanisms and overall structure of thedrum, since it will be possible to contract and expand the drum with asimultaneous movement of all the sectors present.

The Applicant however observes that a correct application of theelementary semi-finished products on the forming drum cannot occurwithout thrust actions transmitted, e.g. by rollers or other applicatormembers, on the elementary semi-finished product in the direction of theabutment surface. According to the perception of the Applicant, in thiscircumstance the presence of the surface discontinuities determined bythe cavities on the abutment surface could cause sudden and uncontrolledvariations of the stresses transmitted by the elementary semi-finishedproduct during the application, with consequent risk of deformation,damage or breakage of the elementary semi-finished products themselvesduring deposition.

The Applicant has moreover perceived that the risks of deformation orbreakage of the elementary semi-finished products during deposition canbe effectively limited or eliminated by suitably modulating the size andgeometric distribution of the cavities on the abutment surface.

In particular, the Applicant has found that, for the purpose of acorrect deposition of the elementary semi-finished products, it isadvantageous to manage the distribution of the cavities in a manner suchthat, at the axially opposite circumferential edges of the abutmentsurface, the transverse width of the solid portions of the abutmentsurface is suitably limited.

More particularly, according to a first aspect, an expandable toroidalforming drum for building tyres forms the object of the presentinvention. Preferably circumferentially consecutive sectors are providedthat are radially moveable between a contracted condition in which saidsectors are approached with respect to a geometric rotation axis of theforming drum, and an expanded condition in which the sectors areradially moved away from said geometric axis in order to define aradially external abutment surface.

Preferably, in the expanded condition, the abutment surface hascircumferential rows of solid portions alternated with hollow portions.Preferably, the solid portions arranged along axially oppositecircumferential edges of the abutment surface, have a transverse sizecomprised between 10% and 60% of a transverse size presented by thesolid portions arranged in proximity to an axial centreline plane of theabutment surface.

In accordance with a second aspect, the invention relates to a processfor building tyres.

Preferably provision is made for building a carcass sleeve.

Preferably provision is made for shaping said carcass sleeve accordingto a toroidal configuration.

Preferably provision is made for engaging a toroidal forming drum withinthe shaped carcass sleeve, for supporting the carcass sleeve against anabutment surface externally provided by the forming drum.

Preferably provision is made for applying at least one elementarysemi-finished product of said tyre around the shaped carcass sleeve, bypressing said elementary semi-finished product towards the abutmentsurface.

Preferably said abutment surface has circumferential rows of solidportions alternated with hollow portions.

Preferably the solid portions arranged along axially oppositecircumferential edges of the abutment surface, have a transverse sizecomprised between 10% and 60% of a transverse size presented by thesolid portions arranged in proximity to an axial centreline plane of theabutment surface.

The Applicant has observed that during the execution of the spiraling,when the drum is supported and suitably moved in order to manage thedistribution of the elementary semi-finished products, the thrust actionexerted for example by an applicator roller tends to push the elementarysemi-finished product together with the underlying parts of the carcassstructure into the hollow portions of the abutment surface, towards theinterior of the corresponding cavities. The Applicant has also perceivedthat while, in the axially internal zones of the abutment surface, thestructural consistency of the carcass structure is able to sufficientlyresist the penetration into the cavity, this may not occur with the sameeffectiveness in proximity of the axially external zones, which lie onthe axially opposite edges of the abutment surface. The Applicant has infact observed that in the axially internal zones, the carcass structureabuts against two axially contiguous solid portions of the abutmentsurface, substantially acting like a bridge-like beam between twosupports, which resists the bending towards the interior of theunderlying hollow portion. At the axially opposite circumferential edgesof the drum, however, the carcass structure projects cantilevered fromthe abutment surface and therefore one of the abovementioned supportscomes to be lacking. The thrust exerted by the applicator roller thustends to deform the carcass structure, making it “sink” into thecavities encountered on the abutment surface during the application. Theroller or other applicator member consequently tends to impact againstthe edges of the solid portions progressively encountered along thecircumferential extension of the abutment surface, generating irregularand discontinuous stresses that can damage the structural components ofthe tyre during manufacturing and make it extremely difficult if notimpossible to execute the spiraling.

This circumstance is more evident in the processing of tyres with a highcurvature ratio, typically seen for example in tyres for motorcycles orother two-wheel vehicles. In a forming drum with high curvature ratio,indeed, the abutment surface has, at a given radial plane, a continuousvariable orientation from the zones close to the axial centreline planeof the drum, where the abutment surface is substantially parallel to therotation axis thereof, to close to the axially opposite edges of theabutment surface, where the orientation of the latter is significantlytilted towards a substantially radial direction. Consequently, thecavities closest to the axially opposite edges of the forming drumgenerate, on the abutment surface, hollow portions of greater size thanthat generated by cavities of equal axial size, close to the axialcentreline plane.

The Applicant deems that by conferring a limited transverse size to thesolid portions defining the axially opposite edges of the abutmentsurface it is possible to ensure a correct application of the elementarysemi-finished products, even when such application is executed byspiraling the same on the carcass structure supported by the formingdrum.

In at least one of the aforesaid aspects, the invention comprises one ormore of the following preferred characteristics that are describedhereinbelow.

Preferably, the solid portions arranged along axially oppositecircumferential edges of the abutment surface have a transverse sizecomprised between 20% and 50% of a transverse size presented by thesolid portions arranged in proximity to an axial centreline plane of theabutment surface.

Preferably, the transverse size of the solid portions is detectable in aradial plane of the forming drum.

Preferably, each sector has circumferentially opposite couplingportions, each comprising elongated projections alternated withcircumferentially extended cavities, in which the projections of eachsector are slidably engaged in the respective cavities ofcircumferentially adjacent sectors.

Preferably, said solid portions and hollow portions are respectivelydefined on said projections and by said cavities.

Preferably, each of said cavities is axially delimited between lateralwalls of two axially consecutive projections.

Preferably, at least some of said projections have substantiallyplate-like structure and lie according to planes parallel to acircumferential extension direction of the abutment surface.

Preferably, said lateral walls are extended according to planessubstantially perpendicular to a geometric rotation axis of the formingdrum.

Preferably, at least some of said cavities are axially delimited, eachbetween the lateral walls of two axially consecutive projections.

Preferably, in each sector, projections belonging to one of saidcoupling portions are offset with respect to the projections belongingto the other coupling portion.

Preferably, axially terminal cavities arranged along the axiallyopposite circumferential edges of the abutment surface are eachdelimited (for example in a radial section plane of the forming drum),between a base surface and a lateral wall respectively converging, bothcarried by an axially terminal projection arranged along saidcircumferential edge.

Preferably, said lateral wall is oriented according to a planesubstantially perpendicular to the geometric rotation axis of theforming drum.

Preferably, said base surface is oriented in a manner substantiallyparallel to the geometric rotation axis of the forming drum.

Preferably, first axially terminal projections arranged along theaxially opposite circumferential edges of the abutment surface areinterconnected, substantially on the entire length, each with an axiallyadjacent projection.

Preferably, first axially terminal projections arranged along theaxially opposite circumferential edges of the abutment surface each have(for example in a radial section plane of the forming drum), a basesurface converging with respect to a lateral wall of a axially adjacentprojection, for delimiting one of said cavities between two solidportions of the axially contiguous abutment surface.

Preferably, second axially terminal projections arranged along theaxially opposite circumferential edges of the abutment surface each havea respectively converging base surface and a lateral wall, delimiting arespective solid portion of the abutment surface.

Preferably, second axially terminal projections arranged along theaxially opposite circumferential edges of the abutment surface eachhave, at the respective base surface, a substantially archedlongitudinal edge, still more preferably convex.

Preferably, in the contracted condition, the projections are inserted inthe cavities according to a measure at least equal to 80% of theirlength.

Preferably, in the expanded condition the projections are extracted fromthe cavities according to a measure at least equal to 80% of theirlength.

Preferably, at least in proximity to an axial centreline plane (“E”) ofthe forming drum (23), each projection has an axial size comprisedbetween about 4 mm and about 15 mm.

Preferably, the hollow portions in the expanded condition have acircumferential size comprised between about 30 mm and about 60 mm.

Preferably, radial movement devices are provided for simultaneouslymoving the sectors between the contracted condition and the expandedcondition.

Preferably, said radial movement devices comprise transmissionmechanisms operatively engageable by actuator devices and configured forsimultaneously translating the sectors from the contracted condition tothe expanded condition.

Preferably, said transmission mechanisms comprise driving levers eachhinged to one of said sectors and to at least one driving collarslidably fit along a central shaft.

Preferably, the driving collar is operatively connected to a threadedbar rotatably engaged in the central shaft.

Preferably, two driving collars are provided that are slidably engagedon the central shaft in axially opposite positions with respect to thesectors, and engaging the threaded bar at respective right hand and lefthand threads.

Preferably, the sectors are carried by respective telescopicallyextendable guide members, radially extending from a central shaft.

Preferably, the forming drum has, in expanded condition, a curvatureratio comprised between about 0.15 and about 0.45.

Preferably deposition devices are provided for applying one or moreelementary semi-finished products in radially external position withrespect to said abutment surface.

Preferably, said deposition devices comprise a driving unit for drivingin rotation the forming drum around its geometric rotation axis and atleast one applicator member for at least one elementary semi-finishedproduct at said abutment surface.

Preferably, said applicator member comprises at least one idle rolleroperating on the elementary semi-finished product in a thrust relationtowards the abutment surface.

Preferably, said forming drum is removably engageable in a shapingstation comprising engagement devices for engaging a carcass sleevecoaxially around the forming drum.

Preferably, shaping devices are also provided, operating in the shapingstation for shaping the carcass sleeve according to a toroidalconfiguration.

Preferably, actuator devices are provided, operating in the shapingstation for radially expanding the forming drum within the carcasssleeve.

Preferably, the actuator devices comprise a rotary driving deviceoperatively engageable with the threaded bar at a first end of thecentral shaft.

Preferably, said deposition devices operate in at least one beltstructure application station.

Preferably, said deposition devices operate in at least one tread bandapplication station.

Preferably, the following are provided:

a carcass building line;

carcass loading devices configured for transferring the carcass sleevefrom the carcass building line to the shaping station.

Preferably, the hollow portions of each circumferential row arecircumferentially offset with respect to the hollow portions of axiallyadjacent circumferential rows.

Preferably, said carcass sleeve comprises at least one carcass ply and apair of annular anchoring structures engaged at axially opposite ends ofsaid at least one carcass ply

Preferably, the carcass sleeve engaged with the forming drum has axiallyopposite terminal flaps projecting cantilevered with respect to theabutment surface.

Preferably, the elementary semi-finished product is applied according toaxially contiguous circumferential coils, in order to form a componentof said tyre.

Preferably, the elementary semi-finished product is pressed against theabutment surface by a localised thrust action against a surface portionof said elementary semi-finished product.

Preferably, the thrust action is exerted by pressing an applicatorroller against the elementary semi-finished product while the formingdrum rotates around a geometric rotation axis thereof.

Preferably, the elementary semi-finished product is pressed by alocalised thrust action in an action area having transverse size smallerthan the transverse size of the hollow portions.

Preferably, the transverse size of the action area is measurable againstthe abutment surface in a radial plane of the forming drum.

Further characteristics and advantages will be clearer from the detaileddescription of a preferred but not exclusive embodiment of a formingdrum, and a process for building tyres, in accordance with the presentinvention.

Such description will be set forth hereinbelow with reference to theenclosed drawings, provided only for exemplifying and hence non-limitingpurposes, in which:

FIG. 1 schematically shows a top view of a plant for building tyres;

FIG. 2 schematically shows, in side view and partial section, theloading of a carcass sleeve on a shaping station;

FIG. 3 schematically shows, in side view and partial section, theengagement of the carcass sleeve with shaping devices arranged in theshaping station;

FIG. 3a shows an enlargement of the detail indicated with “A” in FIG. 3;

FIG. 4 schematically shows, in side view and partial section, theexecution of the shaping of the carcass sleeve;

FIG. 5 shows, in perspective view, several sectors of a forming drum ina contracted condition;

FIG. 6 shows the sectors of FIG. 5 in an expanded condition;

FIGS. 7 and 8 show a single sector of the forming drum seen inperspective view from respectively opposite angles;

FIG. 9 shows the application of a belt layer on the shaped carcasssleeve and coupled to the forming drum;

FIG. 10 shows the application of a tread band on a belt structurecoupled to the carcass sleeve;

FIG. 11 schematically shows, in radial half-section, a tyre obtainablein accordance with the present invention.

With reference to the abovementioned figures, reference number 1indicates overall a plant for building tyres for vehicle wheels. Theplant 1 is arranged to actuate a process according to the presentinvention.

The plant 1 is set for manufacturing tyres 2 (FIG. 11) essentiallycomprising at least one carcass ply 3 preferably internally covered by alayer of impermeable elastomeric material or so-called liner 4. Twoannular anchoring structures 5, each comprising a so-called bear core 5a preferably carrying an elastomeric filler 5 b in radially externalposition, are engaged at respective ends 3 a of the carcass ply/plies 3.The annular anchoring structures 5 are integrated in proximity to zonesnormally identified with the term “beads” 6, at which the engagementbetween the tyre 2 and a respective mounting rim (not depicted) normallyoccurs.

A belt structure 7 is circumferentially applied around the carcassply/plies 3, and a tread band 8 is circumferentially superimposed on thebelt structure 7. Two sidewalls 9, each extended from the correspondingbead 6 to a corresponding lateral edge of the tread band 8, are appliedin laterally opposite positions on the carcass ply/plies 3.

The plant 1 comprises a carcass building line 10 having one or morebuilding positions 11 where the manufacturing of a carcass sleeve 12,having substantially cylindrical shape, is executed for exampleaccording to known modes. The carcass sleeve 12 comprises said at leastone carcass ply 3, preferably internally covered by the liner 4, andhaving respective axially opposite edges ends 3 a engaged, for exampleby turning up, with the respective annular anchoring structures 5. Ifnecessary, the carcass sleeve 12 can also comprise the sidewalls 9 orfirst portions thereof, each extended starting from a respective bead 6.

The carcass building line 10 leads to a shaping station 13 comprisingdevices 14 for engaging the carcass sleeve 12 and shaping devices 15,upon whose action the carcass sleeve 12 is shaped according to atoroidal configuration.

The engagement devices 14 for example comprise a first flange element 16a and a second flange element 16 b, coaxially facing each other andhaving respective circumferential engagement seats 17 a, 17 b, by meansof which they are each operatively engageable at one of the annularanchoring structures 5 respectively carried by the axially opposite endsof the carcass sleeve 12.

The engagement devices 14 can also comprise axial movement members 18 ofthe flange elements 16 a, 16 b. More in detail, it can be provided thatat least one of the flange elements 16 a, 16 b, for example the firstflange element 16 a, is carried by a carriage 19 movable along one ormore linear guides 20, parallel to a geometric axis X-X of mutualalignment between the flange elements 16 a, 16 b and preferably integralwith respect to a fixed base 21, carrying the second flange element 16b. The movement of the carriage 19 along the linear guides 20 determinesthe switching of the shaping station 13 between a loading/unloadingcondition and a work condition. In the loading/unloading condition (FIG.2), the first flange element 16 a is spaced from the second flangeelement 16 b according to a greater measure, approximately at leastdouble, with respect to an axial size of the non-shaped carcass sleeve12, coming from the carcass building line 10. In the work condition, theflange elements 16 a, 16 b, and more precisely the respectivecircumferential engagement seats 17 a, 17 b thereof, are mutually spacedaccording to a measure substantially corresponding to the axial size ofthe carcass sleeve 12.

The shaping devices 15 can for example comprise a fluid-dynamic circuit(not shown) for introducing pressurised air or another operativeinflation fluid between the flange elements 16 a, 16 b, inside thecarcass sleeve 12.

The shaping devices 15 can also comprise one or more linear actuators orother axial movement devices 22, operating on one or preferably both theflange elements 16 a, 16 b in order to move them axially towards eachother starting from the aforesaid work condition. The mutual approachingof the flange elements 16 a, 16 b causes a mutual approaching of theanchoring annular structures 5 so as to allow the shaping of the carcasssleeve 12 according to a toroidal configuration, assisted by thesimultaneous introduction of the pressurised operative fluid into thecarcass sleeve 12.

In the shaping station 13, the shaped carcass sleeve 12 is coupled to atoroidal forming drum 23, substantially rigid and expandable, arrangedinside the carcass sleeve itself.

In FIGS. 1 to 4, the forming drum 23 is only schematically displayed,while in FIGS. 5 to 10 it is depicted in greater detail.

The forming drum 23 is expandable between a radially contractedcondition (FIGS. 2 and 3 and 5), and a radially expanded condition(FIGS. 4, 6, 9 and 10). For such purpose, the forming drum 23 comprisesa plurality of sectors 24 circumferentially distributed around a centralshaft 25. The sectors 24 are movable upon action of radial movementdevices 35, preferably simultaneously with each other, from theaforesaid contracted condition in which they are close to the centralshaft 25, to the expanded condition in which said sectors 24 are movedaway from the central shaft 25. For such purpose, it can be providedthat the sectors 24 are carried by respective telescopically extendableguide members 26, radially extending from the central shaft 25.

Preferably, the contracted condition and expanded condition of thesectors 24 respectively correspond with a maximum radial contractioncondition and a maximum radial expansion condition of the forming drum23.

The movement of the sectors 24 can be attained by means of transmissionmechanisms 27 comprising for example driving levers 28 that are hinged,each at the respectively opposite ends thereof, to one of said sectors24 and to at least one driving collar 29 slidably fit along the centralshaft 25. More particularly, a pair of driving collars 29 are preferablyprovided, situated along the central shaft 25 in axially oppositepositions with respect to the sectors 24, each engaging respectivedriving levers 28.

Each driving collar 29 is operatively connected to a threaded bar 30,rotatably engaged coaxially inside the central shaft 25. The threadedbar 30 is extended along the central shaft 25, nearly for the entirelength thereof or beyond, and carries two axially opposite threads 30 a,30 b, respectively right hand and left hand. Operatively engaged on thethreads 30 a, 30 b are respective nut screws 31, axially movable insidethe central shaft 25 and each connected to one of the driving collars29, e.g. by means of at least one block 32 radially traversing thecentral shaft 25 at a longitudinal slit 33.

The rotation of the threaded bar 30 in the central shaft 25, actuatableby means of a rotary driving device 34 or actuator devices of anothertype operating in the shaping station 13, causes an axial movement ofthe nut screws 31 and the driving collars 29, to which a radial movementof the sectors 24 corresponds, towards the contracted condition or theexpanded condition in accordance with the rotation sense of the threadedbar 30.

In the expanded condition, the set of sectors 24 of the forming drum 23defines, along the circumferential extension thereof, a radiallyexternal abutment surface “S”, toroidally shaped according to theinternal configuration that a part of the carcass sleeve 12 must assumeupon completed shaping. More in detail, it can advantageously beprovided that the abutment surface “S” of the forming drum 23 in theexpanded condition has a curvature ratio comprised between about 0.15and about 0.45, typically adapted for obtaining tyres for motorcycles orother two-wheel vehicles. If necessary, however, curvature ratios can beemployed of values lower than those indicated above, for example adaptedfor producing tyres for cars or trucks.

As illustrated in FIGS. 7 and 8, each of the sectors 24 hascircumferentially opposite coupling portions 36 a, 36 b, preferablyinterconnected by means of an intermediate portion 36 c which has, atleast on the abutment surface “S”, a main extension direction parallelto a radial plane of the forming drum 23. In FIGS. 1 to 4, the geometricaxis of the drum coincides with the geometric axis X-X.

Each of the coupling portions 36 a, 36 b has a plurality of elongatedprojections 37 extended in circumferential direction from theintermediate portion 36 c, alternated with respective circumferentiallyextended cavities 39.

In a same sector 24, the projections 37 belonging to one of the couplingportions, for example 36 a, are offset with respect to the projections37 of the other coupling portion 36 b.

At least one part of the projections 37, more precisely at least theprojections 37 situated at the axially internal zones of the drum, ornot situated along the axially opposite circumferential edges thereof,can have substantially plate-like structure, and lie according to planesparallel to a circumferential extension direction of the abutmentsurface “S”. Such projections 37 therefore have lateral walls 38extended according to planes orthogonal to the geometric rotation axisof the drum.

At least the cavities 39 situated at the axially internal zones of thedrum, or not situated at the axially opposite circumferential edgesthereof, are axially delimited, each between lateral walls 38 of twoaxially consecutive projections 37.

As is better illustrated in FIGS. 5 and 6, the projections 37 of eachsector are slidably engaged in the respective cavities 39 of thecircumferentially adjacent sectors 24, and are adapted to slide in thecavities themselves in order to support the expansion and contractionmovements of the forming drum 23.

The lateral walls 38 of the respectively mating projections 37 belongingto circumferentially contiguous sectors 24 mutually guide the sectorsthemselves during the contraction and expansion movement, and facilitatethe maintenance of a satisfactory structural solidity of the formingdrum 23 overall, both in the contracted condition and in the expandedcondition.

In the contracted condition, the projections 37 of each sector 24penetrate into the respective cavities 39 until they touch or nearlytouch the intermediate portion 36 c of the adjacent sector 24. Moreparticularly, in the contracted condition, the projections 37 areinserted in the respective cavities 39 according to a measure at leastequal to 80% of their length.

In the expanded condition, the projections 37 are extracted from thecavities 39 according to a measure at least equal to 80% of theirlength.

The presence of the projections 37, of the cavities 39 and their mutualrelation ensure that, on the abutment surface “S”, circumferential rowsof solid portions 40 are identifiable, at least in the expandedcondition, defined by the projections 37, alternated with hollowportions 41 defined by the cavities 39. The solid portions 40 and hollowportions 41 belonging to each circumferential row are circumferentiallyoffset with respect to the solid portions 40 and, respectively, hollowportions 41 of axially adjacent circumferential rows.

It is opportune that the surface discontinuities induced by thealternation of solid portions 40 and hollow portions 41 does notcompromise a correct execution of the obtainment of the tyre 2components during building. For such purpose, it is preferably providedthat at least in proximity to an axial centreline plane “E” of theforming drum 23, still more preferably over all the projections 37except for those situated along the axially opposite circumferentialedges of the forming drum 23, each projection 37 has axial sizeapproximately comprised between about 4 mm and about 15 mm, preferablyequal to about 8 mm. Each cavity 39 has preferably axial size equal tothat of the projections 37 circumferentially aligned therewith.

Axial size larger than the indicated values could be excessive for thepurpose of a correct support of the carcass sleeve 12 and/or of othercomponents of the tyre 2, also in consideration of the stressestransmitted during processing. Axial size smaller than the indicatedvalues could in turn lead to excessive structural complexity of thesectors 24, with consequent increase in the production costs, inaddition to possible structural weakening.

It is also preferably provided that the hollow portions 41 in theexpanded condition have a circumferential size comprised between about30 mm and about 60 mm, preferably equal to about 40 mm.

At the axially opposite circumferential edges of the abutment surface“S”, the axial size of the projections 37 and cavities 39 indicatedabove can be excessive for the purpose of a correct processing.

For such purpose, it should be observed that due to the curvatureprovided by the cross section profile of the abutment surface “S”, thetransverse size of the solid portions 40 and hollow portions 41 is notequal to the axial size of the corresponding projections 37 and cavities39 to which they belong. In particular, in the forming drums intendedfor obtaining tyres for two-wheel vehicles, where the curvature ratio isrelatively accentuated, the transverse size of the solid portions 40 andhollow portions 41 at the axially opposite circumferential edges can beseveral times larger than that which can be encountered in proximity tothe axial centreline plane “E”.

Consequently the axially opposite terminal flaps 12 a of the carcasssleeve 12, projecting cantilevered from the opposite circumferentialedges of the abutment surface “S”, can be overly weak and freely movablefor the purpose of opposing the stresses.

It is therefore preferable that the axial size of the projections 37 andcavities 39 at the axially opposite circumferential edges of theabutment surface “S” are different from those detectable in theremaining axial more internal portions of the forming drum 23. Moreparticularly, the sizing of the projections 37 and cavities 39 isselected and modulated on the width of the abutment surface “S”, in amanner such that the circumferential rows of solid portions 40 definingaxially opposite circumferential edges of the abutment surface “S”, orarranged in proximity of the same edges, have a transverse size “T1”approximately comprised between 10% and 60%, more preferably between 20%and 50%, of a transverse size T2 presented by the circumferential rowsof solid portions 40 arranged in proximity to an axial centreline planeof the abutment surface “S”. Such transverse size is detectable alongthe profile of the abutment surface “S”, in a radial plane of theforming drum 23, and in the described embodiment can assume, at theaxially opposite circumferential edges of the abutment surface “S”, avalue on the order of about 2 mm.

Values lower than the indicated minimum thresholds could induceexcessive localised stresses, in particular on the elastomericcomponents radially facing towards the tyre 2 interior, directly incontact on the abutment surface “S”. Values greater than the indicatedmaximum thresholds could excessively reduce the effect of a suitablesupport for the terminal flaps 12 a of axially opposite ends of thecarcass sleeve 12.

It is thus possible to suitably limit the cantilever according to whichthe axially opposite terminal flaps 12 a of the carcass sleeve 12,carrying the beads 6, project with respect to the abutment surface “S”.In other words, notwithstanding the discontinuities provided by theabutment surface “S”, the axial ends of the sleeve 6 are suitablysupported for the purpose of the subsequent obtainment of the componentsof the tyre 2, as will be better described hereinbelow.

It is preferably provided that the axially terminal projections,identified with 37 a, 37 b arranged along the axially oppositecircumferential edges of the abutment surface “S”, are interconnectedsubstantially along the entire length thereof detectable in thecircumferential direction, with the axially adjacent projections 37belonging to the same coupling portion 36 a, 36 b.

More particularly, in the coupling portions 36 a, 36 b of each sector24, a first axially terminal projection 37 a and a second axiallyterminal projection 37 b are respectively identifiable, along each ofthe axially external circumferential edges. The first axially terminalprojection 37 a has, in a radial section plane of the forming drum 23, abase surface 42 a converging with respect to the lateral wall 38 of anaxially adjacent projection 37, preferably at right angle, so as todelimit one of said cavities 39, identified here as axially terminalcavity 39 a, between two of the axially contiguous solid portions 40 ofthe abutment surface “S”.

The second axially terminal projection 37 b, substantially complementaryto the axially terminal cavity 39 a, in turn has a base surface 42 b anda lateral wall 38 respectively converging, preferably at right angle,delimiting a respective solid portion 40 of the abutment surface “S”.More particularly, the lateral wall 38 is preferably oriented accordingto a plane perpendicular to the geometric rotation axis of the formingdrum 23. The base surface 42 b can in turn be oriented parallel to thegeometric rotation axis of the forming drum 23.

In passing from the expanded condition to the contracted condition, thesectors 24 simultaneously translate towards the geometric rotation axisof the forming drum 23, mutually approaching each other according to asubstantially circumferential direction. In order to support such mutualmovement without jamming due to mechanical interferences, it ispreferably provided that the second axially terminal projections 37 beach have, at the respective base surface 42 b, a convex longitudinaledge 43, extended according to a substantially arched profile.

Preferably, the forming drum 23 is positioned in the shaping station 13before the respective carcass sleeve 12, e.g. still being processedalong the carcass building line 10, reaches the shaping station itself.

More particularly, it is preferably provided that the forming drum 23 besupported cantilevered in the shaping station 13. For example, a firstend, 25 a of the central shaft 25 of the forming drum 23 can for suchpurpose be retained by a mandrel 44 coaxially housed in the first flangeelement 16 a and carrying the aforesaid rotary driving device 34couplable with the threaded bar 30 in order to drive it in rotation.

The forming drum 23 can therefore be arranged in the contractedcondition by means of said rotary driving device 34, if it is notalready situated in such condition upon reaching the shaping station 13.

By means of carcass loading devices 45, the carcass sleeve 12 comingfrom the carcass building line 10 is then transferred into the shapingstation 13 in order to be subsequently coaxially arranged in radiallyexternal position around the forming drum 23 arranged in the contractedcondition.

The carcass loading devices 45 can for example comprise a carcasshandling device 46 preferably operating on an external surface of thecarcass sleeve 12. With a radial translation movement (with respect tothe forming drum 23), the carcass sleeve 12 is first inserted, in axialalignment relation with the forming drum 23, between the flange elements16 a, 16 b arranged in the loading/unloading condition (FIG. 2). Thecarcass sleeve 12 is subsequently arranged around the forming drum 23,preferably following an axial translation movement of the forming drum23 itself. More particularly, with a movement of the carriage 19 alongthe linear guides 20, the forming drum 23 is coaxially inserted in thecarcass sleeve 12.

Preferably, the translation of the carriage 19 and the forming drum 23terminates with the engagement of a second end 25 b of the central shaft25 with a tailstock 47, situated inside the second flange element 16 b(dashed line in FIG. 2).

For the axial movement of the forming drum 23 with respect to thecarcass sleeve 12 to occur without mutual mechanical interferences, itis preferably provided that in the contracted condition, the formingdrum 23 has a maximum external diameter smaller than a minimum internaldiameter of the carcass sleeve 12, typically detectable at the beads 6.

At the end of the axial movement, each of the anchoring annularstructures 5 integrated in the beads 6 is situated in axially internalposition with respect to the circumferential engagement seat 17 a, 17 bof the respective first and second flange element 16 a, 16 b.

Upon action of the axial movement devices 22, the flange elements 16 a,16 b then carry the respective engagement seats 17 a, 17 b substantiallyin radial alignment relation inside the annular anchoring structures 5.

Each of said flange elements 16 a, 16 b comprises expansion members (notdepicted) configured for determining a radial expansion of respectivecircumferential seal rings 48 a, 48 b integrating the circumferentialengagement seats 17 a, 17 b. Following such radial expansion, each ofthe circumferential seal rings 48 a, 48 b is brought to act in thrustrelation against one of the anchoring annular structures 5. The carcasssleeve 12 is thus stably constrained to the flange elements 16 a, 16 b.Upon completed engagement, the carcass handling device 37 can disengagethe carcass sleeve 12 and be removed from the shaping station 13.

During shaping, when the carcass sleeve 12 starts to radially expand,the radial expansion of the shaping drum can be driven by means ofrotation of the threaded bar 30 upon action of the rotary driving device34.

The coupling between the carcass sleeve 12 and the forming drum 23 isthus enabled. Such coupling occurs by bringing an internal surface ofthe carcass sleeve 12 in contact relation against the abutment surface“S” of the forming drum 23.

In order to facilitate an expansion of the carcass sleeve 12, it can beprovided that in the final steps of approaching the maximum radialexpansion condition of the carcass sleeve 12, the flange elements 16 a,16 b are axially inserted in radially internal position with respect tothe sectors 24 of the forming drum 23, which is about to reach theexpanded condition.

Upon completed coupling, the flange elements 16 a, 16 b disengage thecarcass sleeve 12, leaving it on the forming drum 23.

Carcass sleeve 12 and forming drum 23 in mutual coupling relation areadapted for being subjected to the action of the deposition devices 49,in order to form components of the tyre 2 being processed by means ofapplication of one or more elementary semi-finished products in radiallyexternal position with respect to the abutment surface “S”.

The deposition devices 49 can for example comprise at least one device50 for building at least one belt layer in radially external positionwith respect to the shaped carcass sleeve 12. Such device 50 ispreferably installed in a belt structure application station 51 that isremote with respect to said shaping station 13.

In order to allow the transfer of the forming drum 23 to the beltstructure application station 51, it is provided that the forming drum23 carrying the carcass sleeve 12 is supported by the mandrel 44operating at the first end 25 a of the central shaft 25, while thetailstock 47 is disengaged from the second end 25 b of the central shaft25 itself. With a retreat of the first flange element 16 a, the shapingstation 13 is brought back into the loading/unloading condition, freeingthe access for an anthropomorphic robotic arm 52 or other suitabledriving unit, which in turn engages the forming drum 23 at the secondend 25 b of the central shaft 25.

The robotic arm 52 transfers the forming drum 23 from the shapingstation 13 to the belt structure application station 51. The robotic arm52 also suitably moves the forming drum 23 in front of the belt layerbuilding device 50, which can for example comprise a dispenser 53 whichfeeds at least one elementary semi-finished product 54 a, e.g. in theform of a rubber-covered cord or another continuous elongatedreinforcement element made of textile or metallic material. Preferablyassociated with the dispenser 53 is a preferably idle roller 55, oranother suitable applicator member for the elementary semi-finishedproduct 54 a at the radially external surface of the tyre 2 beingprocessed. The roller 55 operates in thrust relation against a surfaceportion of the elementary semi-finished product 54 a, pressing ittowards the abutment surface “S” in order to determine the applicationthereof according to axially contiguous circumferential coils, on thecarcass sleeve 12 or other radially underlying element. For example, abelt layer 7 a (at 0 degrees) can thus be obtained by winding theelementary semi-finished product in rubber-covered cord form accordingto axially adjacent circumferential coils around the abutment surface“S”, while the forming drum 23 is driven in rotation and suitably movedby the robotic arm 52.

The rigidity of the forming drum 23 ensures a stable positioning of thesingle circumferential coils 44 formed directly on the external surfaceof the shaped carcass sleeve 12, without undesired deformations of thecarcass sleeve 12 occurring due to the stresses transmitted on itsexternal surface during application. The stickiness of the greenelastomeric material that constitutes the carcass ply or plies 3prevents undesired spontaneous and/or non-controlled movements of thesingle circumferential coils 44, without having to arrange for thispurpose additional intermediate layers between the belt layer 7 a inmanufacturing step and the underlying application surface. In otherwords, a precise positioning is facilitated of the singlecircumferential coils 44 of the belt layer 7 a, directly formedaccording to the desired final profile of the carcass sleeve 12 uponcompleted shaping, even when such profile has an accentuated transversecurvature as is for example found in tyres intended for motorcycles ortwo-wheel vehicles.

The belt structure application station 51 can if necessary comprisedevices 57 for building one or more auxiliary layers 7 b, to be appliedon the shaped carcass sleeve 12 before or after the application of saidat least one belt layer 7 a. In particular, such auxiliary layers 7 bcan comprise textile or metallic parallel cords, arranged according toan orientation that is tilted with respect to the circumferentialextension direction of the carcass sleeve 12, respectively crossedbetween auxiliary layers 7 b adjacent to each other.

By means of the robotic arm 52, or by means of a second anthropomorphicrobotic arm or handling device of another type, the forming drum 23 isthen transferred from the belt structure application station 51 to atread band application station 58, preferably constituting part of agreen tyre completion line integrating the same belt structureapplication station 51.

In the tread band application station 58, a spiraling unit 59 can forexample operate, configured for winding at least one elementarysemi-finished product 54 b in the form of a continuous elongated elementmade of elastomeric material according to circumferential coils, axiallyadjacent in mutual contact, at a radially external position around thebelt structure 7, while the forming drum 23 is driven in rotation andsuitably moved, e.g. by the same robotic arm 52, for distributing thecircumferential coils according to a predefined scheme. The applicationof the elementary semi-finished product 54 b in the form of a continuouselongated element also in this case occurs with the aid of a respectivesecond applicator roller 60 operating in a manner analogous to thatdescribed with reference to the obtainment of the belt layer 7 a. Theplant 1 can also comprise devices for obtaining sidewalls (notillustrated) against axially opposite lateral portions of the carcasssleeve 12, which can operate in a manner analogous to the spiraling unit59.

The geometric and size parameters of the projections 37, and cavities39, arranged in the forming drum 23 allow suitably supporting thecarcass sleeve 12 without this undergoing excessive distortions orlocalised stresses under the effect of the thrust exerted by theapplicator roller 55, 60. Indeed, at each of the cavities 39, thecarcass sleeve 12 is supported as a bridge between two axiallycontiguous projections 37. In this situation, the carcass sleeve 12 isadapted to act as a kind of bridge-like beam sustained between twosupports, suitably opposing the thrust action exerted by the applicatorroller 55, 60, even if such thrust is localised in an action area,measurable against the abutment surface “S” in a radial plane of theforming drum 23, having transverse size smaller than the transverse sizeof the hollow portions 41. This circumstance can for example occur whenthe thrust action of the roller 55 is concentrated on an elementarysemi-finished product 54 having size smaller than the transverse size ofthe hollow portions 41, as is exemplified in FIG. 9, or when thetransverse size of the hollow portions 41 is larger than the axial sizeof the applicator roller 60, as is exemplified in FIG. 10.

Indeed, illustrated in FIG. 10 is the application of the continuouselongated element for the purpose of obtaining the tread band 8. Due tothe limited transverse size of the solid portions generated by the firstaxially terminal projections 37 a, the extent to which terminal flaps 12a of the carcass sleeve 12 cantilever project from the abutment surface“S” is suitably limited.

It is thus possible to effectively apply tread band 8 and/or thesidewalls 9 up to close to the beads 6, without the carcass sleeve 12having excessive structural yielding under the thrust action exerted bythe applicator roller 60.

The built green tyre 2 is suitable for being removed from the formingdrum 23 in order to then be vulcanised in a vulcanisation unit 61.

1-37. (canceled)
 38. An expandable toroidal forming drum for buildingtyres, comprising: circumferentially consecutive sectors radiallymoveable between a contracted condition in which said sectors areapproached with respect to a geometric rotation axis of the formingdrum, and an expanded condition in which the sectors are radiallymoveable away from said geometric axis in order to define a radiallyexternal abutment surface, wherein, in the expanded condition, theabutment surface has circumferential rows of solid portions alternatedwith hollow portions, and wherein the solid portions arranged alongaxially opposite circumferential edges of the abutment surface have atransverse size between 10% and 60% of a transverse size presented bythe solid portions arranged in proximity to an axial centreline plane ofthe abutment surface.
 39. The drum as claimed in claim 38, wherein thesolid portions arranged along axially opposite circumferential edges ofthe abutment surface have a transverse size between 28% and 50% of atransverse size presented by the solid portions arranged in proximity toan axial centreline plane of the abutment surface.
 40. The drum asclaimed in claim 38, wherein the transverse size of the solid portionsis detectable in a radial plane of the forming drum.
 41. The drum asclaimed in claim 38, wherein each sector has circumferentially oppositecoupling portions, each comprising elongated projections alternated withcircumferentially extended cavities, wherein the projections of eachsector are slidably engaged in respective cavities of circumferentiallyadjacent sectors.
 42. The drum as claimed in claim 41, wherein saidsolid portions and hollow portions are respectively defined on saidprojections and by said cavities.
 43. The drum as claimed in claim 41,wherein each of said cavities is axially delimited between lateral wallsof two axially consecutive projections.
 44. The drum as claimed in claim41, wherein at least some of said projections have substantiallyplate-like structure and lie according to planes parallel to acircumferential extension direction of the abutment surface.
 45. Thedrum as claimed in claim 43, wherein said lateral walls are extendedaccording to planes substantially perpendicular to a geometric rotationaxis of the forming drum,
 46. The drum as claimed in claim 43, whereinat least some of said cavities are axially delimited, each between thelateral walls of two axially consecutive projections.
 47. The drum asclaimed in claim 41, wherein, in each sector, projections belonging toone of said coupling portions are offset with respect to projectionsbelonging to the other coupling portion.
 48. The drum as claimed inclaim 41, wherein axially terminal cavities arranged along the axiallyopposite circumferential edges of the abutment surface are eachdelimitated between a base surface and a lateral wall, respectivelyconverging, both carried by an axially terminal projection arrangedalong said circumferential edge.
 49. The drum as claimed in claim 48,wherein said lateral wall is oriented according to a plane substantiallyperpendicular to the geometric rotation axis of the forming drum. 50.The drum as claimed in claim 48, wherein said base surface is orientedin a manner substantially parallel to the geometric rotation axis of theforming drum.
 51. The drum as claimed in claim 41, wherein first axiallyterminal projections arranged along the axially opposite circumferentialedges of the abutment surface are interconnected, substantially on anentire length, each with an axially adjacent projection.
 52. The drum asclaimed in claim 41, wherein first axially terminal projections arrangedalong the axially opposite circumferential edges of the abutmentsurface, each have a base surface converging with respect to a lateralwall of an axially adjacent projection, for delimiting one of saidcavities between two axially contiguous solid portions of the abutmentsurface.
 53. The drum as claimed in claim 41, wherein second axiallyterminal projections arranged along the axially opposite circumferentialedges of the abutment surface, each have a base surface and a lateralwall, respectively converging, delimiting a respective solid portion ofthe abutment surface.
 54. The drum as claimed in claim 41, whereinsecond axially terminal projections arranged along the axially oppositecircumferential edges of the abutment surface, each have, at arespective base surface, a substantially arched longitudinal edge. 55.The drum as claimed in claim 41, wherein, in the contracted condition,the projections are inserted in the cavities according to a measure atleast equal to 80% of a length of the cavities.
 56. The drum as claimedin claim 41, wherein, in the expanded condition, the projections areextracted from the cavities according to a measure at least equal to 80%of a length of the cavities.
 57. The drum as claimed in claim 41,wherein, at least in proximity to an axial centreline plane of theforming drum, each projection has an axial size between about 4 mm andabout 15 mm.
 58. The drum as claimed in claim 42, wherein the hollowportions in the expanded condition have a circumferential size betweenabout 30 mm and about 60 mm.
 59. The drum as claimed in claim 38,further comprising radial movement devices for simultaneously moving thesectors between the contracted condition and the expanded condition. 60.The drum as claimed in claim 59, wherein said radial movement devicescomprise transmission mechanisms operatively engageable by actuatordevices and configured for simultaneously translating the sectors fromthe contracted condition to the expanded condition.
 61. The drum asclaimed in claim 60, wherein said transmission mechanisms comprisedriving levers, each driving lever hinged to one of said sectors and toat least one driving collar slidably fit along a central shaft.
 62. Thedrum as claimed in claim 61, wherein the driving collar is operativelyconnected to a threaded bar rotatably engaged in the central shaft. 63.The drum as claimed in claim 61, comprising two driving collars slidablyengaged on the central shaft in axially opposite positions with respectto the sectors, and engaging the threaded bar at respective right handand left hand threads,
 64. The drum as claimed in claim 38, wherein thesectors are carried by respective telescopically extendable guidemembers, radially extending from a central shaft.
 65. The drum asclaimed in claim 38, having, in expanded condition, a curvature ratiobetween about 0.15 and about 0.45.